As known from prior art, the petroleum industry uses processes for the conversion of heavy hydrocarbon charges wherein the hydrocarbon molecules with a high molecular weight and with a high boiling point are split up into smaller molecules that are capable of boiling at lower temperature ranges, depending on the desired application.
To effect this type of conversion, the petroleum industry uses, in particular, so-called fluid-state cracking processes. In these types of processes, the hydrocarbon charges, in generally pulverized in the form of small droplets, is put in contact with cooling particles at high temperature and which circulate in the reactor in the form of a fluidized bed, i.e., in a more or less dense suspension within a gaseous fluid which ensures or assists in its transport. In contact with the hot particles, the charge vaporizes, and the hydrocarbon molecules are cracked. [The cracking reaction is a thermal reaction in case the particles only have a cooling function.] The cracking reaction is catalytic by nature in case the cooling particles also have a catalytic function, i.e., they represent active sites promoting the cracking reaction, as is the case, in particular, in the so-called fluid-state catalytic cracking process (commonly referred to as FCC process, based on the English “Fluid Catalytic Cracking”).
After reaching, upon completion of the cracking reactions, the desired range of molecular weight combined with a corresponding reduction of the boiling point, the reaction effluents are separated from the particles. The latter, deactivated under the influence of the coke which has deposited on their surface, are generally stripped in order to recover the hydrocarbons carried along, then regenerated by combusting the coke, and finally once again put in contact with the charge to be cracked.
The reactors used are most frequently tubular-type vertical reactors in which the charge and the particles move in an essentially rising flow (in which case the reactor is then called a “riser”) or in an essentially downward flow (in which case the reactor is then referred to as a “dropper” or “downer”).
One major difficulty which such processes encounter is simultaneously cracking the charge both completely and selectively, i.e., to succeed in cracking the entire charge in order to obtain a maximum quantity of valuable hydrocarbons while minimizing the quantity of undesirable byproducts. This object is even more difficult to attain considering that the charges to be cracked have relatively wide boiling point ranges and contain very different components which crack under significantly different conditions to produce a variety of products.
For that reason, the processes currently in use lead to generally incomplete conversion of the charge. With these processes, cracking is performed in a single reactor whose operating conditions, chosen depending of the average nature of the hydrocarbons making up the charge, do not make it possible to properly crack the entire range of hydrocarbons present to selectively obtain the desired products. As a result, reaction effluents are obtained which contain very different products, a significant percentage of which are the result of insufficient cracking of the charge and which represent undesired, difficult-to-use products for the refiner.
A first solution consists in recycling all or part of the products obtained as a result of the cracking reaction in order to reprocess them in a second cracking stage. Such a measure, however, is not only very inefficient, but also detrimental, insofar as a result of such recycling, the cracking quality of the fresh charge is frequently notably affected.
A second solution consists in increasing the cracking intensity to more comprehensively crack the charge injected and convert all types of hydrocarbons that are present. Such a measure, however, although making it possible to increase the conversion rate of the charge, in turn promotes overcracking phenomena, which translate to a decrease in conversion selectivity: an increased production of dry gases and coke is observed, to the detriment of the desired intermediate hydrocarbons.
Several solutions have been proposed in prior art to overcome the above-mentioned difficulties.
Since 1947, U.S. Pat. No. 2,488,713 has been proposing a catalytic cracking process using two successive reactors in each of which catalytic particles circulate. In the first reactor, a heavy recycled cut (a residue resulting from the fractionation of the cracking effluents, of the type known by the name “slurry”) is cracked in contact with catalytic particles from a regenerator. In the second reactor, a fresh charge as well as an intermediate recycled cut of the distillate type are cracked in contact with particles from the first reactor. At the outlet of either of the two reactors, the hydrocarbonated effluents are stripped of particles, then combined and directed towards a conventional fractionating column.
The first disadvantage of such a process is that the fresh charge is treated, in the second reactor, in the presence of particles which have already been largely coked and deactivated in the first reactor, in contact with the heavy recycled charge, which is particularly rich in resistant polyaromatic components. As a result, in the second reactor, these particles perform poorly in terms of catalytic activity, which leads to mediocre cracking of the fresh charge, while producing at the same time a low conversion rate and poor selectivity.
A second disadvantage is due to the fact that the heavy recycled cut is progressively enriched with the most resistant heavy components which, even if they are recycled in the first reactor, do not crack at all or only incompletely, and “go around and around” in the unit. This aggravates the problems described above in terms of premature coking of the particles in the first reactor. Purging in the recycle line does not resolve this problem in a satisfactory manner. As a matter of fact, since the recycled cut consists of the fractionating residue of the combined effluents of the two reactors, purging not only extracts only a part of the most resistant components which are supposed to be removed from the unit, but also additionally extracts a fraction of the components directly coming from the fresh charge which have not been converted while passing into the second reactor, but which could have been cracked in the first reactor in contact with the regenerated particles. The poor selectivity of this purging system therefore causes an additional loss of yield in terms of desired products.
In addition, EP No. 573316 describes a catalytic cracking process wherein the reaction occurs in two successive reactors, the first reactor being a downer, and the second a riser. The charge to be cracked is brought into contact with regenerated particles at the inlet of the downer, at the bottom of which the charge/particle mixture is transferred to the riser. The charge then circulates in contact with the particles in the two successive reactors, which makes it possible to increase the overall yield of cracked hydrocarbons. However, this process is not fully selective: hydrocarbons already converted in the first reactor are once again cracked in the second reactor, which leads to an overcracking phenomenon, resulting in increased production of dry gases and coke, to the detriment of the desired intermediate cuts.
In the pursuit of its research in the field of fluidized bed cracking, the Applicant has become interested in processes in which two cracking reactors are used in order to improve the rate and selectivity of the conversion as compared with traditional processes using only one single reactor. In the process, the Applicant has developed a process which makes it possible to overcome the disadvantages of prior art systems.